libostree: Get and set OstreeAsyncProgress:status atomically

Use the new well-known `status` key for OstreeAsyncProgress to get and
set the status atomically with other keys in an OstreeAsyncProgress
instance.

Signed-off-by: Philip Withnall <withnall@endlessm.com>

Closes: #819
Approved by: cgwalters

src/libostree/ostree-repo-pull.c

@@ -240,10 +240,10 @@ update_progress (gpointer user_data)
                              /* We fetch metadata before content.  These allow us to report metadata fetch progress specifically. */
                              "outstanding-metadata-fetches", "u", pull_data->n_outstanding_metadata_fetches,
                              "metadata-fetched", "u", pull_data->n_fetched_metadata,
+                             /* Overall status. */
+                             "status", "s", "",
                              NULL);
 
-  ostree_async_progress_set_status (pull_data->progress, NULL);
-
   if (pull_data->dry_run)
     pull_data->dry_run_emitted_progress = TRUE;